4 Analysis of the photospheres of the target stars

Prior to searching for diffuse band, molecular and atomic absorptions in the spectrum of Star 6, it was necessary to fully characterise the photospheric component of the spectrum. We used the Kurucz ATLAS9 model atmosphere grids (Kurucz 1991) to perform a detailed abundance analysis and ultimately (for Star 6 only) spectral synthesis. For clarity of the text, the details on the derivation of the atmospheric parameters of Star 6 from the UVES data are given in the Appendix, and only the results are reported here.

Firstly, the heliocentric radial velocity for Star 6 was found to be $+52.4 \pm 0.6$ km s^-1. This implies that photospheric components of e.g. atomic resonance lines (Na  I, K  I) are shifted well away from any absorptions arising in the IRC +10$\degr$ 216 CSE at $v_{\rm helio}=-19.3$ km s^-1.

Secondly, the best-fit parameters for Star 6 were found to be: $T_{\rm eff} = 5650$ K, $\log g = 4.3$ (cgs), $V_{\rm turb} = 1.3$ km s^-1 and $\rm [Fe/H] = -0.4$ dex, relative to solar. Abundances for other elements are given in the Appendix. These results confirm that Star 6 is a low metallicity object of spectral type G, perhaps somewhat cooler than the Sun, with a very similar surface gravity. The metallicity suggests that Star 6 is likely to be a member of the thick disk population. Most importantly, adopting a luminosity class V yields an absolute magnitude $M_{\rm v}=+5.3$ for Star 6 (Lang 1991), suggesting a distance of 1400 pc and confirming that Star 6 is indeed well beyond IRC +10$\degr$ 216.

Photospheric parameters and a spectroscopic distance have also been derived for Star C, yielding $T_{\rm eff}\sim 6250$ K, $\log g \sim 4.8 \pm 0.2$, $V_{\rm turb}\sim 2.1$ km s^-1 and $\rm [Fe/H] \sim-0.4$ dex relative to solar. The heliocentric velocity is near 0 km s^-1. If luminosity class V, Star C has spectral type near F8, and $M_{\rm v} = +4.0$ yields a distance of 480 pc. If Star C is a subdwarf rather than a solar-type dwarf, then its absolute magnitude is $\sim$1.3 mag fainter (Allen 1973), hence the distance would be reduced by a factor of 1.8 to 260 pc, still twice the accepted distance of IRC +10$\degr$ 216.

Figure 2: K  I lines in the VLT spectrum of Star 6, shown in heliocentric velocity space. The solid line is the K  I 7665 Å  profile, the dashed line K  I 7699 Å. The three vertical lines trace circumstellar components in K  I, centred very closely to the heliocentric velocity of IRC +10$\degr$ 216 at -19.3 km s-1 .